Their main reason for this decision is performance: some of Facebook’s repositories are so large that they are bringing Git to its knees. They looked at the Git source code and the Mercurial source code and felt that the latter would be easier to fine tune to give them the performance that they needed.

This is an interesting development. With Git now on the verge of overtaking Subversion to the most widely used SCM in corporate settings, it’s tempting to write off Mercurial as something of a lost cause. Back in January, when I posted a suggestion on the Visual Studio UserVoice forums that Microsoft should support Mercurial as well as Git in Team Foundation Server, I thought it would be doing well to get three hundred votes and then plateau. But as it stands, it’s now passed 2,500 votes and still going strong, making it the tenth most popular open request on the forums and the second most popular request for TFS in particular, with nearly twice as many votes as the original DVCS request had. Git may have cornered the market for open source collaboration, but its unnecessarily steep learning curve and often pathological behaviour make it surprisingly unpopular with the majority of developers for whom public collaboration on open source projects is not a priority.

It’ll be interesting to see the outcome of this, but one thing is for certain: it’s not all over yet.